Just to remind now that many of us with TRVZBs are in the heating season that many boilers feature an internal thermostat to automatically start so as to prevent freeze up damage to the boiler. This will need a path for the circulating heating water to flow when the TRVZBs are in ‘Keeping’ status. We achieve this with one radiator fitted with a thermo-electric actuator controlled by a Sonoff Basic Switch and arranged by scenes to be open when every TRVZB in the system is in ‘Keeping’ status.
Normal practice in the UK is to leave one radiator without a TRV for this reason - usually a bathroom radiator. Heated towel rails are great for this.
There is another ‘normal’ practice here in the UK: a bypass loop between flow and return with a gate valve for example to restrict the flow through the loop. This has the disadvantage of ‘shunting’ some of the circulating water away from the radiators. The main disadvantage of leaving one rad fully open is that it will heat up if any other radiator is calling for heat even if the room when heat is not needed in that room- wasting energy. The way I see it is if we are designing smart heating systems using valves like the TRVZBs then the whole system should be smart and not rely on legacy practices that were being used with systems that had simple or no controls.
True, however if you go for the heated towel rail option then I find this less of an issue, because the heat is helping dry my towels. A TRV won’t measure the dampness of your towels and call for heat, that is unless someone can do something clever with a repurposed soil moisture meter?
I would probably go for the same option if we had a radiator in the bathroom but ours is underfloor heating. Sonoff do make some temperature and humidity devices in Zigbee and Wi-Fi. We use their TH16 to switch a bathroom extractor fan on and off- works fine to clear the room from steam and condensation automatically when using a shower.